The invention relates to square prism filters, and more particularly to transmitter combiners including a plurality of contiguous square prism filters.
Transmitter combiners are devices which allow simultaneous transmission of signals from a plurality of transmitters at different closely spaced frequencies by means of a single antenna. Transmitter combiners include a number of tuned cavities, one corresponding to each transmitter and each frequency. Each bandpass (or band reject) filter is coupled by a coaxial cable to a separate respective transmitter and is also coupled to a common coaxial connector to which the single antenna is connected. Until recently, the vast majority of transmitter combiners in use have been constructed of coaxial tuned cavities, rather than square prism filters (cavities) because most mobile communication systems have operated in assigned 150 megahertz and 450 megahertz bands. The sizes of square prism filters operable at these frequencies are so large that it was more practical (from a space savings point of view) to use coaxial tuned cavities than square prism filter cavities. More recently, an 880 megahertz band has been allocated for mobile telephone communications. In this band, square prism filters having dimensions of approximately 9 inches by 9 inches by 3 inches are practical. Holders of FCC licenses in this band have established "cells" or regions in major metropolitan areas, each cell being typically several miles in extent, each having a low power antenna that, generally is centered in that cell in a major metropolitan areas. Recent rapid growth of the mobile telecommunication market has greatly increased the number of antennas needed. Antenna sites in metropolitan areas are very expensive. Therefore, there is a great deal of incentive to provide small, compact transmitter combiners for use at such antenna sites. Although it would seem that square prism filters, due to their rectangular parallel piped structure, could more easily be arranged in space saving configurations than coaxial tuned filters, the fact is that it has always been necessary to tune square prism filters from controls located on the outer geometric center portions of a square face of a square prism filter. This has prevented "stacking" square prism filters together or building large multiple cavity devices with shared square walls. Positioning of the tuning controls for square prism filters near the center of the square walls has been necessitated by the fact that the tuning rods or elements should extend into a portion of the cavity near, or at least aligned with the geometric center of the cavity in order to be effective and in order to provide adequate tuning control without introducing unacceptably large amounts of insertion loss. Those skilled in the art know that, as a general matter, insertion of any conductive element into a tuned cavity causes insertion loss which, of course, must be minimized in any state of the art tuned cavity device.
It is clear that there is an unmet need for a compact multiple tuned cavity device and, more particularly, for compact high performance transmitter combiners.